Abstract

A study was made at U.S. Steel Corp.'s Minntac open-pit taconite mine in northern Minnesota to assess the applicability of relating magnetic susceptibility measurements made in production blast holes to the magnetite content of the strata penetrated. The measurements were made by continuously recording the changes in self-inductance of variously constructed air-core, multiturn coils as they were raised in the holes. These coils were designed so that the magnetic flux which coupled with the wallrock was parallel to the horizontally bedded Biwabik iron-formation. An analysis of the response for these coils showed that the recorded inductance changes (apparent magnetic susceptibilities) are related linearly to the average weight percent Fe as magnetite within a practical degree of accuracy. In one test hole, 36 assayed intervals, each 18 inches long, fitted a linear line of regression to the corresponding apparent magnetic susceptibilities within a standard error of + or -1.2 weight percent Fe. When averaged over three ranges of Fe content, the values fitted a straight line to within + or -0.5 weight percent Fe. As susceptibility depends on the mode of occurrence of the magnetite, extrapolation of these results to other areas will require further testing. This method of assay should provide control for the efficient processing of taconite ores.Since the method is rapid and inexpensive, measurements could be made in scores of these accessible holes to delineate cut-off boundaries between ore and waste.A potential use of these continuous measurements would be found in establishing lithologic contacts from the detailed responses in selected horizons of the iron-formation.

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